This application claims priority to German Patent Application No. 101 51 787.4-16, filed Oct. 19, 2001, which is incorporated by reference herein.
The present invention relates to a heat exchanger system for a device for autothermal reforming of a hydrocarbon featuring a reaction zone, which is fed with at least two fluids which react with one another. The reaction zone is at a higher temperature level than the environment of the device, and at least one of the fluids is heated by the reaction products effluent from the reaction zone. The heat exchanger system includes tubes for the effluent reaction products. These tubes are arranged essentially parallel to a common center axis at least in one tube section and led through the flow path of the fluid to be heated.
Heat exchanger systems of this kind are employed in systems that are used to produce a hydrogen-rich gas from liquid hydrocarbons. The catalytic conversion of liquid hydrocarbons to a hydrogen-rich gas, the so-called xe2x80x9creformate gasxe2x80x9d, is carried out in several successive steps, the actual reforming and a subsequent staged shift reaction. During reformation, the hydrocarbons are broken down into H2, CO and CO2 in accordance with the thermodynamic equilibrium. During the subsequent shift reaction, CO and H2O are catalytically converted to CO2 and H2.
The individual reactions of such autothermal reforming take place at different temperature levels which, moreover, are matched to the catalysts used in each case. Thus, the actual reforming takes place at about 800-900xc2x0 C., a high-temperature shift at about 400xc2x0 C., and a low-temperature shift at about 200xc2x0 C. To achieve as high a system efficiency as possible, the heat of the reformats gas is coupled back into the system and used via a heat exchanger system for preheating the educts required for the autothermal reforming.
A heat exchanger system of that kind which is designed as a counter-current heat exchanger is known from German Published Patent Application No. 197 27 84 1. Here, the heat exchanger is essentially composed of individual shells which are formed of tubes that are arranged concentrically around the reaction zone. Air, oxygen, water vapor and, possibly, water are fed into the reaction chamber from outside to inside via individual annular gaps between the shells while the reformate gas is conveyed from inside to outside via adjoining annular gaps in counter-current while exchanging heat. In this manner, heat losses toward the outside are minimized. In this heat exchanger system, the energy transfer is essentially based on thermal radiation. Because of the flow cross-sections, only low flow velocities and, consequently, poor heat transfer are achieved so that energy transfer by convection has hardly any effect here.
U.S. Pat. No. 3,955,941 describes a different heat exchanger system, which takes advantage of the fact that the heat transfer is very efficient at a tube onto which the flow impinges transversely. Here, the heat exchanger system is a so-called xe2x80x9cshell-and-tube heat exchangerxe2x80x9d in which the reformate gas is removed from the reaction zone via tubes which are arranged parallel to one another. The air required for the reforming passes between the tubes into reaction zone and is thereby heated. To this end, the air stream is directed to the tubes as perpendicularly as possible using baffles. This always involves a loss of pressure, which is more or less strong, depending on the design of the baffles. In practice, the Boudouard reaction, in which CO is converted to CO2 and elemental carbon can occur during the cooling of the reformate gas. This reaction is temperature-dependent and is described as follows:
CO2+C⇄2CO 
The elemental carbon deposits as soot and can then no longer be used for the production of H2. This not only reduces the system efficiency but also results in unwanted contamination of the device. The probability of occurrence of the Boudouard reaction increases with the dwell time of the flowing gas in the heat exchanger and with increasing temperature. Thus, care must be taken that the reformate gas does not cool too slowly after the reforming step.
From the further related art, there are known heat exchanger systems which, however, were designed for other purposes of use. Thus, German Patent Application DE 196 01 579 A1 and German Patent DE 196 50 086 C1 show systems for exchanging thermal energy between large gas/vapor flows and small liquid flows. In this context, the gas/vapor phase flows in concentrically arranged annular channels, which are equipped with axial, liquid-carrying tubes.
German Utility Model Patent DE 81 09 730 U1, which also belongs to the general prior art and, at its heart, is related to the filtering of oil, describes a heat exchanger in which the oil is led spirally around cooling tubes by means of baffles.
The heat exchanger system proposed by the present invention allows efficient exchange of heat and, consequently, efficient cooling of the effluent reaction products as, for example, of the reformate gas, combined with a short dwell time of the reaction products in the heat exchanger system. To this end, at least sections of the flow path of the fluid to be heated are curved helically or spirally around the center axis of the tube arrangement so that the fluid to be heated impinges on and passes around the tubes containing the effluent reaction products essentially perpendicularly.
The present invention provides a heat exchanger system for a device for autothermal reforming of hydrocarbons featuring a reaction zone, which is fed with at least two fluids which react with one another, the reaction zone being at a higher temperature level than the environment of the device, and at least one of the fluids being heated by the reaction products effluent from the reaction zone. The heat exchanger includes tubes for the effluent reaction products, the tubes being arranged essentially parallel to a common center axis at least in one tube section and led through the flow path of the fluid to be heated. At least sections of the flow path are curved helically or spirally around the center axis of the tube arrangement so that the fluid to be heated impinges on and passes around the tubes containing the effluent reaction products essentially perpendicularly. The flow path of the fluid to be heated is laid out with respect to the center axis of the tube arrangement in such a manner that the fluid to be heated is conveyed from outside to inside and/or from inside to outside at least once.
The efficient cooling of the reaction products is attributable to the fact that all heat transfer mechanisms available here for the transfer of heat, namely radiation, convection and heat conduction are made use of through the structural measures according to the present invention. Thus, for example, the heat exchanger system according to the present invention allows direct transfer of radiant heat from the effluent reaction products to the fluid to be heated. Therefore, it is also possible to use tubes having a relatively small flow cross-section for the effluent reaction products, resulting in a correspondingly high flow velocity and, consequently, in a short dwell time of the reaction products in the heat exchanger system. Since in this case, the pressure load on the hot-gas side is also relatively low, it is possible to use more cost-effective (i.e. cheaper) materials for the implementation of the heat exchanger system according to the present invention. Finally, a further advantage to be mentioned is that the heat exchanger system according to the present invention can be implemented in a very compact form of construction.
In the heat exchanger system according to the present invention, which features a particularly compact design and a good exchange of heat, the flow path of the fluid to be heated is laid out with respect to the center axis of the tube arrangement in such a manner that the fluid to be heated is conveyed from outside to inside and/or from inside to outside at least once. In the case of a helically curved flow path, the fluid can, for example, be initially led helically upward around the tubes in the outer region of the tube arrangement and then be led helically downward around the tubes in the inner region of the tube arrangement and fed into the reaction zone. In the case of a spirally curved flow path, the fluid can, for example, be led spirally around the tubes from outside to inside at the upper end of the tube arrangement and then be led back from inside to outside at a lower level of the tube arrangement and so on until the fluid reaches the bottom end of the tube arrangement and is fed into the reaction zone.
In an advantageous variant of the heat exchanger system according to the present invention, the flow cross-section is essentially constant in the entire flow path of the fluid to be heated. In this case, the heat transfer between the effluent reaction products and the fluid to be heated is particularly efficient.
To allow as good a flow impingement as possible on the tubes for the effluent reaction products and, consequently, to achieve as good a heat transfer as possible to the fluid to be heated, it turns out to be advantageous for the tubes to be arranged in a staggered manner in the flow path of the fluid to be heated.
In principle, there are different possibilities for the structural implementation of the heat exchanger system according to the present invention. If the tube arrangement is arranged in a housing, a shell-type design turns out to be advantageous in which the flow path for the fluid to be heated is bounded by at least one partition wall in the housing that is oriented essentially parallel to the center axis of the tube arrangement and is curved around the center axis. The partition walls for a helically curved flow path can simply be formed of tubes of different diameters that are arranged concentrically around the center axis. Moreover, the flow path for the fluid to be heated is additionally bounded by partition plates, which are helically designed and arranged between the partition walls. The partition wall for a spirally curved flow path is curved spirally around the center axis. Moreover, the flow path for the fluid to be heated is additionally bounded by the partition plates, which are oriented essentially perpendicularly to the center axis of the tube arrangement.